1. Field of the Invention
The present invention relates to an electrophotographic photoconductor which comprises a conductive support, an undercoating layer and a photosensitive layer, and to a method for manufacturing the same.
2. Description of the Prior Art
Generally speaking, the process of electrophotography is one means for recording data using a photoconductive phenomenon observed in a photoconductor. The process of electrophotography is conducted in the following way.
At the outset, the photoconductor is placed in a dark place to be electrostatically charged homogeneously on the surface thereof by corona discharge, followed by exposing an image to selectively discharge an electric charge at an exposing section so that an electrostatic image is formed at a non-exposed section.
Subsequently, colored electrically charged fine particles (toner) are allowed to adhere to the electrostatic image by electrostatic force or the like to form a visible image, thereby forming an electrophotographic image.
Basic properties required for the photoconductive photoconductor for use in electrophotographic technique which undergo these series of processes include the following points:
(1) The photoconductor can be homogeneously charged to an appropriate level of potential in a dark place.
(2) The photoconductor has a high electric charge holding capabilities and only a small amount of electric discharge.
(3) The photoconductor has a high photosensitivity such that irradiating the photoconductor with light causes a quick discharge of an electric charge.
In addition, the photoconductor requires good stability and durability such as:
(4) The electrostatic charge on the photoconductor can be easily removed.
(5) The residual potential is small.
(6) The photoconductor has a mechanical strength and an good flexibility.
(7) In the case of repetitive use, electric properties, particularly charging properties, photosensitivity, residual potential and the like vary little.
(8) The photoconductor has resistance against heat, light, temperature, moisture and ozone deterioration.
Electrophotographic photoconductors currently put on the market as a product are constituted by forming a photosensitive layer on a conductive support. Besides, an undercoating layer is provided between the conductive support and a photosensitive layer for the following purposes:
Inhibiting the generation of image defect resulting from the disappearance and reduction of an electric charge on the surface of the photosensitive layer caused by unnecessary injection of an electric charge into the photosensitive layer from the conductive support, PA1 Coating defects on the surface of the conductive support, PA1 Improving the charging properties, PA1 Improving the adhesiveness of the photosensitive layer, and PA1 Improving the coating properties of the photoconductor. PA1 dispersing non-conductive titanium oxide particles and a polyamide resin into a mixed solvent of a lower alcohol selected from the group consisting of methanol, ethanol, isopropyl alcohol and n-propyl alcohol and an organic solvent selected from the group consisting of chloroform, 1,2-dichloroethane, dichloromethane, trichlene, carbon tetrachloride, dimethylformamide and 1,2-dichloropropane, PA1 applying the resulting mixture to a conductive support to form an undercoating layer, and PA1 forming a photoconductive layer on the undercoating layer.
Resins to be used for the undercoating layer include resin materials such as polyethylene, polypropylene, polystyrene, acrylic resin, vinyl chloride resin, vinyl acetate resin, polyurethane resin, epoxy resin, polyester resin, melamin resin, silicon resin, polyvinyl butyral resin and polyamide resin, copolymer resin containing two or more of the above repetitively used units such as vinyl chloride-vinyl acetate copolymer resin, acrylonitrile-styrene copolymer resin, caseine, gelatin, polyvinyl alcohol, ethyl cellulose. Particularly, polyamide resin is said to be preferable (Japanese Laid-Open Patent Publication No. SHO 48-47344, Japanese Laid-Open Patent Publication No. SHO 52-25638, and Japanese Laid-Open Patent Publication No. SHO 58-95351).
The electrophotographic photoconductor using polyamide resin or the like for the undercoating layer thereof has a resistance of about 10.sup.12 to 10.sup.15 .OMEGA..multidot.cm with the result that the residual potential is accumulated in the photosensitive layer to generate an overlap of images without reducing the thickness of the undercoating layer to about 1 .mu.m or less. On the other hand, there was a problem that reducing the thickness of the undercoating layer results in making it difficult to control the thickness of the undercoating layer in the process such that defects on the conductive support cannot be coated and the charging properties of the photoconductor cannot be improved.
In addition, polyamide resin having a favorable adhesiveness with metal cannot be dissolved in general organic solvents. Thus it has an excellent resistance against solvent with respect to the photosensitive layer. On the other hand, it has a drawback that it absorbs a large amount of moisture with the result that the residual potential rises in low temperature and low moisture conditions under the influence of the large moisture absorption.
Further, it has a drawback that the residual potential is accumulated in large quantity and the photosensitivity is reduced in the repetitive use so that image is overlapped, causing a damage to the quality of the image.
Besides, in order to inhibit the image defect and to improve the residual potential, there has been proposed an electrophotographic photoconductor in which is provided an undercoating layer having 1 to 10 weight part of a mixture of titanium oxide and tin oxide-scattered into 100 weight part of 8-nylon (as disclosed in Japanese Laid-Open Patent Publication No. SHO 62-280864) and an electrophotographic photoconductor using titanium oxide fine particles coated with alumina for improving dispersing properties of the titanium oxide (as disclosed in Japanese Laid-Open Patent Publication No. HEI 2-181158).
Thus it has become possible to increase the thickness of the undercoating layer by mixing titanium oxide in the undercoating layer, but there was a problem that the stability in the repetitive use depends on the environmental conditions particularly in low temperature and low moisture environments.
Consequently, it is important to select the most appropriate polymer material out of a large number of such materials in order to provide an electrophotographic photoconductor excellent in repetitive stability and environmental properties wherein the residual potential is accumulated in small amount and photosensitivity reduces a little in repetitive use by providing an undercoating layer between the conductive support and the photosensitive layer to improve the charging properties and residual potential of the photoconductor. That is because when the photosensitive layer contacts the undercoating layer, a charge generation material may come together to cause a failure in coating, thereby generating a drawback of reduction in photosensitivity and uneven quality of images. In addition, resins and metal oxides used in the undercoating layer must be stable both in the combination and the ratio of blend without causing a change in resistance by environmental conditions such as low temperature low moisture and high temperature high moisture. Further, such resins and metal oxides must form a block against a hole injection from the conductive support as well as exhibit a resistance against solvents in the process of forming a photosensitive layer.